Structure of excited states in ${}^{21}\mathrm{Mg}$ studied in one-neutron knockout

Bound excited states in the neutron-deficient nucleus ${}^{21}\mathrm{Mg}$ were probed in the one-neutron knockout reaction ${}^9\mathrm{Be}({}^{22}\mathrm{Mg},{}^{21}\mathrm{Mg}+\gamma )\mathrm{X}$ at 74 MeV/nucleon projectile energy. The low-lying level scheme of ${}^{21}\mathrm{Mg}$ was investigated for the first time with $\gamma$-ray spectroscopy. Contrary to the interpretation of previous particle spectroscopy data, our proposed excitation scheme is in agreement with the mirror nucleus and shell-model calculations in the $p\text{−}\mathit{sd}$ shell. Spectroscopic factors for the one-neutron removal from ${}^{22}\mathrm{Mg}$ to ${}^{21}\mathrm{Mg}$ are extracted and compared to shell-model calculations using the WBP effective interaction.

Figure 1

Experimental level schemes for the mirror nuclei ${}^{21}\mathrm{F}$ (a) and ${}^{21}\mathrm{Mg}$ (c) [8], compared to results from a WBP shell-model calculation (b). In (d) we show the ${}^{21}\mathrm{Mg}$ decay scheme as proposed in the present work. All energies are in MeV, only states below 3.7 MeV are shown.

Figure 2

Particle identification of reaction residues in coincidence with incoming ${}^{22}\mathrm{Mg}$ secondary beam particles. The identification was achieved using the S800 focal-plane detectors [22] as described in the text. The gate used as identification of the $N=9,Z=12\hspace{0.5em}{}^{21}$Mg reaction residue is shown.

Figure 3

Doppler-corrected energy spectrum for $\gamma$ rays emitted by ${}^{21}\mathrm{Mg}$ in the one-neutron knockout from ${}^{22}\mathrm{Mg}$.

Figure 4

Background-subtracted, acceptance-corrected momentum distributions for the one-neutron knockout to ${}^{21}\mathrm{Mg}$. The inclusive momentum distribution (a) and the momentum distributions for knockout to the 1084- and 1989-keV levels (b) and (c) are shown. Theoretical distributions for orbital angular momenta $\ell =0,\ell =1$, and $\ell =2$ are shown as dotted, dashed, and dot-dashed lines, respectively. For the inclusive momentum distribution the individual contributions are added according to the partial cross sections listed in Table . In all cases, the theoretical distributions are scaled to match the amplitude and centroid of the measured distribution.